The present invention relates to a belt transferring device for transferring on a record paper a toner image formed onto an image forming member, such as a photosensitive drum.
In an elecrophotographic printing apparatus or a copying device, an electrostatic latent image is formed onto a photosensitive member or body, the electrostatic latent image is developed to make a toner image, the toner image is transferred onto a recording medium, such as, a record paper and the transferred image is subjected to a fixing step to form a hard copy. Various kinds of transferring devices for transferring the toner image formed onto the photosensitive body are well known, for example, from a belt transferring device disclosed in Japanese Patent Application Laid-open No. 61-117583. In this known belt transferring device, a transfer belt formed by a dielectric material is stretched between two drive rollers, the transfer belt is charged at an even potential by a corona charger, the record paper carried in synchronism is electrostatically attracted onto the transfer belt, and then an image transfer process is performed while carrying the record paper under the state of holding it to the transfer belt. After transferring the toner image onto the record paper, charges on the surface of the transefer belt are removed with A.C. process by a charge removing device which is composed of a high A.C. supply source and a corona charger, after which a cleaning blade is pressed onto the surface of the transfer belt to clean the belt.
Such a transferring device has an advantageous effect that the record paper is electrostatically attracted onto the transfer belt, as well as the transferring and the carrying are performed under the state of holding the record paper onto the transfer belt, thereby obtaining high reliablity for carrying the record paper.
In the above known belt transferring apparatus, however, the surface of the transferring belt is cleaned by depressing a cleaning blade onto the transferring belt so that uneven force is liable to add to the transfer belt and thus the transfer is liable to meander. That is, it is often difficult to depress the cleaning blade onto the running transfer belt with even depressing force, so that the transfer belt becomes meandered even though slightly uneven depression force is effected upon the transfer belt by depressing the cleaning blade. Particularly, in the high speed apparatus the transfer belt is remarkably susceptible of meander by uneven depression force due to high speed running on the transfer belt.
In the belt transferring device, moreover, after transferring and before charging the charges on the transfer belt are removed with A.C. process so as always to cause constant voltage on the transfer belt. In order to remove charges on the transfer belt with A.C. process, corona discharge is performed by using an A.C. high voltage supply source and a corona charger, thereby removing charges on the surface of the transfer belt, so that electric shock and dielectric breakdown or the like are arisen. Moreover, the A.C. high voltage supply source is very expensive, resulting in a high manufacturing cost.